potential drop

[Reepicheep]

Hello

I've got the following problem:

I'm collecting the data of several (3 so far) pressure-sensors with a NI-DAQ (USB6210). The sensor output is in voltage (mV). I had no problem with connecting the sensor and the DAQ, and I also have a clear reading in LabVIEW SignalExpress and in MAX (Measurement & Automation Explorer). So far, everything is ok.

But periodically, I've got a potential drop to 0, alternating with a voltage increasing... I checked every part of the sensorplatform with a voltmeter (sensors, power supply, etc.) and everything works perfectly (no drop and no peak). The failure must happen somewhere at the DAQ...  But I've got no clue why or how I could "repair" it...

I attached an image of an data-aqquisition with LabVIEW SignalExpress, where you can see the problem... (the "short" peaks are due to the experimental set-up and could be removed easily)

Thanks in advance for any help!

Reepicheep

[Reepicheep]

I checked the whole set-up with an USB6251 and received a similar graph...

I've got readings from all three sensor between 20 and 25mV, and all the sudden they jump to exactly 530.04308mV and stay there for about 5 sec. If I check with the Voltmeter right at the DAQ, everything stays around 25mV... So there must be a problem with the software-settings or something similar....

[Reepicheep]

No ideas? no help?

[PeterAdelhardt]

Hi Reepicheep,
It seems like it would be a configuration problem, especially since you verified it with another DAQ device. What measurement configuration are you using? Differential or single ended? How did you connect your signals?

A good page for wiring and measurement setup configurations can be found here.
Best Mode for grounded sources; Best mode for floating sources (battery, etc - grounds not connected).

For the wiring configuration of the 621x, see this manual - p4-24 et seqq.

Please let me know if you were able to measure correctly when adjusting your wiring and settings.

Thanks and best regards,

Peter

[Reepicheep]

Hi Peter

Thanks for your reply!

The measurements before (=> image) have been made with NRSE and 2Hz with sample 1. The signals have been connected without any resistors or ground... Just as it was shown on the connection-plan of the NI-programs...

But since then, I already found page 4-24 of the manual with the help-function. And so I connected everything as shown in the box of "DIFF" and "Groundfree Signalsource". So now, there is no drop anymore, but if got still a wave-function (see new attachment)...

How can I eliminate this one and if possible, also the short peaks?

Thanks in advance,
  Reepicheep

[PeterAdelhardt]

Hi Reepicheep,
I am glad you are making progress with the measurement.

Since your signal amplitude is very small, some noise is bound to be included in the measured signal.

Did you make sure to connect the AI- channel to AIGND? (p 4-27) Did you use Bias resistors?
See also this document for eliminating AC noise. This might be caused by switching power supplies, etc. Your best bet would be to either oversample and average or to filter the signal once you are sure your measurement configuration is correct.

Good luck and best regards,

Peter